Psalm 119 has the perfect verse to direct us to memorize Scripture:
119:11, I have stored up your word in my heart,
that I might not sin against you.
Thomas Manton tells us several good reasons to do this, including that we may have it ready for our use:
When it is hid in the heart, it will be ready to break out in the tongue and practice, and be forthcoming to direct us in every duty and exigency. When persons run to the market for every penny-worth, it doth not become good housekeepers.